Copper powders have been sought for many years for use in electrical applications because of its conductive properties. Copper is also less expensive than other metals (i.e. silver) leading to its wide commercial use.
There has been a considerable research effort to improve the conductivity of copper for microelectronic applications (i.e. adhesives). For this reason, the size of copper powder has been explored for electrical conductivity. In general, copper powder used in conductive adhesives is extremely fine (median particle size D50<10 μm). Fine powder provides more particle contact and consequently higher electrical conductivity.
Fine copper powder is made by inert gas atomization, where molten copper is broken up by high-pressure inert gas which produces individual droplets that solidify into powder particles. Typically, these gas atomized particles are generally spherical in shape. Spherical particles provide a single point of contact with adjacent particles, although several particles may contact a single particle leading to many contact points. It is known that the larger the number of inter-particle contacts, the higher the electrical conductivity of the copper powder.
There are some disadvantages in using fine copper powders with a D50<10 μm. One disadvantage is the cost of production. For example, the atomization process generates powder with a wide range of particle sizes. Selective use of fine particles lowers the yield of usable powder, and thus increases cost as the oversize particles must be either re-melted or scrapped. Another disadvantage is that copper has tendency to oxidize particularly when it is in fine particle form. For example, exposing fine copper powder to air for a short period of time will result in surface oxidation. The oxide form of copper increases contact resistance and reduces electrical conductivity, making it unfit for the some electrical applications i.e. microelectronic adhesives.
Other research efforts to improve the electrical conductivity of copper involve silver-coating copper particles to prevent oxidation. Silver is a highly electrically conductive metal. Even when oxidized, silver does not have substantially altered electrical conductivity. However, coating silver on the surface of copper adds significantly to production costs.
Some prior art references describe chemically treating copper powder with an anti-oxidizing film to improve electrical conductivity. Suitable anti-oxidizing films described are organic acid salts of higher aliphatic amines with 0.2 to 10 parts by weight of a boron-nitrogen dispensing agent and 0.1 to 10 parts by weight of a coupling agent, based on the total weight of the powder. Some coupling agents described are isopropyl-triisostreroyl-titanate and aceoalkoxy-aluminum-diisopropylate.
Other prior art references describe preventing oxidation of copper by surface treating copper powder or silver-coated copper powder with coupling and treating agents. The total amount of coupling and treating agents used is 0.1% to 10% based on the weight of the copper powder. Some coupling agents described are silane, titanate, aluminate and zirconate. One treating agent described in the prior art is ZB-3, which is the reaction product of peanut oil fatty acids, boric acid and triethanolamine.
Copper flakes have also been utilized to improve electrical conductivity. These flakes are said to provide less surface area per unit volume leading to improved particle contact when compared to copper powder. However, the surfaces of these copper flakes are cleaned to remove oxides and/or impurities. One disadvantage associated with copper flakes is that they require special handling to avoid oxidation. For example, typically copper flakes are stored in a nitrogen atmosphere and then quickly incorporated into the adhesive (i.e. epoxy) to preserve electrical conductivity.
Based on the disadvantages above, there is a need for copper powders that have excellent electrical conductivity for microelectronic applications. There is also a need for copper powders that are oxidatively stable, not requiring special handling.